Compositionally Controlled Polyether Membranes via Mono(μ-alkoxo)bis(alkylaluminum)-Initiated Chain-Growth Network Epoxide Polymerization: Synthesis and Transport Properties

The addition of a diglycidyl-ether to a mono(μ-alkoxo)bis(alkylaluminum)-initiated epoxide polymerization presents a strategy for amorphous polyether-based membrane synthesis. In situ kinetic ¹H NMR spectroscopy was used to monitor model network copolymerizations of epichlorohydrin (ECH) with 1,4-butanediol-diglycidyl ether (Butyl-dGE) or poly(ethylene oxide)-diglycidyl ether (PEO-dGE). Reactivity ratios were extracted from the evolution of polymer composition from the monomer feed during copolymerization. Quantitative conversion and nearly random comonomer incorporation was achieved. The generality of this synthetic technique was supported by the polymerization of Butyl-dGE and a range of epoxide monomers such as n-butyl glycidyl ether (nBGE), allyl glycidyl ether, ECH, and glycidol. The copolymerizations produced optically clear, flexible films in all cases. We investigated the potential for this synthetic platform to provide compositional control of structure-property relationships within the context of industrially relevant membrane separations for CO₂. Given the affinity of PEO for CO₂ and water, we explored using nBGE as a hydrophobic diluent, which was copolymerized with varying incorporations of PEO-dGE. The resultant cross-linked polyether membranes exhibited high CO₂ permeabilities (150-300 barrer) and selectivity over N₂ (α_CO2/N2 = 20-30) and H₂ (α_CO2/H2 ≈ 6). CO₂ sorption isotherms could be described by Henry's law and did not vary across the series of nBGE/PEO-dGE films. The similar sorption coefficients suggested that differences in permeability among these samples were driven by differences in diffusion coefficients. The diffusivity of CO₂ increased with cross-link density, and permeability was unaffected by humidity for this series of hydrophobic cross-linked polyether membranes.